Sleeping on your back during late pregnancy can compress a major blood vessel called the inferior vena cava, reducing blood flow to both you and your baby. This becomes a concern primarily after 28 weeks, when the uterus is heavy enough to press against the vein when you’re lying flat. Before that point, the evidence suggests your sleep position doesn’t meaningfully affect pregnancy outcomes.
What Happens Inside Your Body
The inferior vena cava is the large vein that carries blood from your lower body back to your heart. It runs along your spine. When you lie on your back in late pregnancy, the weight of the uterus presses this vein (and the aorta beside it) against your vertebrae, partially flattening it. Research using ultrasound has shown that the vein’s diameter shrinks dramatically when a pregnant woman shifts from lying on her side to lying on her back.
This compression triggers a chain of effects. With less blood returning to the heart, cardiac output can drop by 25% to 30% in the second half of pregnancy. Less blood pumping means less oxygen-rich blood reaching the placenta. Studies measuring fetal oxygen levels found that when a mother lies on her back, fetal oxygen saturation averages about 48%, compared to roughly 52% on the right side and 56% on the left side. The difference is modest, but sustained compression over hours could matter. At the same time, resistance in the uterine artery increases, meaning the blood that does reach the uterus has a harder time flowing through to the placenta.
When It Actually Matters
Most guidance focuses on 28 weeks and beyond. A 2019 study published in Obstetrics and Gynecology found that back sleeping through the 30th week of pregnancy was not associated with higher rates of stillbirth, low birth weight, or preeclampsia. The concern ramps up in the third trimester, when the uterus is at its heaviest and the compression effect is most pronounced.
A large meta-analysis published in The Lancet’s eClinicalMedicine found that going to sleep on the back in late pregnancy was associated with 2.6 times the odds of late stillbirth compared to falling asleep on the left side. The population-level impact was estimated at about 5.8%, meaning that roughly 1 in 17 late stillbirths could theoretically be linked to supine sleep position. These are observational numbers, not proof of direct cause, but they were consistent enough across multiple studies that many obstetric organizations now include sleep position in their late-pregnancy guidance.
Warning Signs Your Body Gives You
Most women won’t lie on their backs for long without noticing something feels off. The drop in blood return to the heart can cause dizziness, nausea, a racing heartbeat, or shortness of breath. Some women feel lightheaded or clammy. This cluster of symptoms is sometimes called supine hypotensive syndrome, and it’s essentially your body telling you to roll over. If you feel any of these sensations while lying on your back, shifting to your side typically resolves them within seconds as blood flow normalizes.
If You Wake Up on Your Back
This is the part that causes the most anxiety, and the reassurance is strong. Briefly ending up on your back is not dangerous. Cleveland Clinic physicians have noted that even an hour or two on your back is unlikely to harm your baby. The studies linking back sleep to adverse outcomes looked at women who spent entire nights on their backs without shifting, which is uncommon in practice. Most people change positions multiple times during sleep.
If you wake up on your back, simply roll to your side. Your body’s discomfort signals generally prevent you from staying supine for extended periods. The goal is to fall asleep on your side, not to maintain perfect positioning all night long, which is impossible to consciously control.
Left Side vs. Right Side
Left-side sleeping has traditionally been recommended because the inferior vena cava sits slightly to the right of the spine, so lying on the left theoretically takes all pressure off the vein. Fetal oxygen measurements do show a small advantage for the left side over the right. However, a large NIH-funded study found that adverse outcomes were no more common among women who slept on the right side compared to the left. About 1,900 pregnancies with complications were analyzed, and position on either side made no meaningful difference to outcomes.
In practical terms, either side is fine. If you naturally prefer your right side, there’s no need to force yourself onto your left. The key distinction is side versus back, not left versus right.
Making Side Sleeping More Comfortable
Staying on your side through the night gets harder as pregnancy progresses. A pillow between your knees helps align your hips and reduces lower back strain. Many women find that a long body pillow or a wedge pillow behind the back prevents unconscious rolling. Placing a small pillow under your belly can also relieve the pulling sensation on your ligaments.
If you can’t sleep flat on your side, even a slight tilt helps. Propping yourself at an angle with pillows behind your back, so you’re not fully supine, reduces the compression on the vena cava. You don’t need to be perfectly perpendicular to the mattress. Any position that keeps the uterus from resting directly on the vein is an improvement over lying completely flat.